The invention relates to a device for the cyclic rearrangement of a pile of rectangular or square pictures, or a so-called "picture-changer".
Picture changers are known from U.S. Pat. Nos. 4,238,898, 4,238,899, 4,241,528, 4,241,529, 4,245,417, 4,259,802 and 4,376,348. These patents are all based on the principle that a pile of pictures, especially photographic prints, is held by two frame parts that are movable relative to one another, one of which has a viewing window. During each complete cycle of movement of the frame parts (for instance in the form of a housing and a slider member), that is, pulling them fully away from each other and pushing them fully back together again, one picture is removed from one end of the pile and returned to the other end of the pile again. The picture changers have the following components for this:
A feeding means which feeds pictures to the separating means;
a separating means which detaches a single picture from the pile;
a retaining means which holds the individual picture separated from the pile in one of the frame parts whilst the remainder of the pile is held in the other frame part;
a guide means which guides the separated individual picture in such a manner that it passes to the other end of the remainder of the pile.
If a picture changer of this type is used for photographic prints, then problems arise as a result of the specific properties of the pictures, which are very thin (of the order of 0.25 mm). They consist of a layered construction of paper and plastics. The result of this is that with changing environmental conditions the pictures become unforeseeably warped and bowed. In addition they tend, as a result of moisture or static charge, to stick together.
During the changeover operation, the cyclic rearrangement of the pile of pictures, if the changeover operation is interrupted for any reason and the user pushes the slider member, when it has been only partially withdrawn, back into the housing, four critical situations arise:
The first critical situation arises if the rear edge (in the direction of movement) of the picture to be rearranged is about to emerge from a separating gap for separating the picture from the remainder of the pile. The guide means has at that point not yet begun to displace this edge transversely to the separator bar, but a part of this edge is already beyond the bar whilst another still lies beneath it (if, for instance, the picture edge in question does not run exactly parallel to the separator bar). This is shortly before the outer end position. If, then, the slider member, instead of being pulled right out is pushed back in, the edge of the picture strikes the separator bar and is buckled, so that the picture can neither be guided back under, nor back over, the pile.
The second critical situation arises shortly before the inner end position is reached. The rear edge (in the direction of movement) of the picture is then still located in the return through-gap whilst the next changeover operation is already being prepared for. Here, too, there is the possibility that the picture might become buckled.
The third critical situation arises if, during the first phase, the withdrawal of the slider member, a part of the separated picture still remains in the separating gap but the next picture in the pile has already been detached and is "waiting" with its leading edge in front of the inlet of the separating gap. If the frame parts are then pushed together the oppositelying edges of the two pictures strike one another and result in buckling.
The fourth critical situation arises if, during the second phase, the operation of insertion of the slider member, operation is interrupted and the slider member is moved back and forth several times by short amounts. The risk here is that the rear "pushed" edge of the individual picture swings loose from the "pushing" component, the picture changer becoming inoperable.